High tension separation is a well established technique for classifying particulate materials such as mineral ores, shredded refuse, seeds and the like. In conventional practice separation is effected according to the electrical conductivities of the constituents of the particulate mixture. Usually, particles are fed to the top of a rotating, grounded, conductive roll or drum and are bombarded with ions from a corona discharge electrode to charge the particles on the roll surface so that they tend to adhere to the surface of the roll. The more conductive particles tend to lose their charge rapidly by conduction and soon separate from the roll under the influence of centrifugal force and the force of gravity. Less conductive particles tend to retain their charge and remain pinned to the roll for a longer period of time so that they can be collected as a separate fraction. Generally, the charged particles are subjected to a static field which assists in drawing away the conductive particles while holding the nonconductive particles to the rotating conductive roll.
Examples of such systems are disclosed in Grave, U.S. Pat. No. 2,072,501; Hewitt, U.S. Pat. No. 2,314,940; Johnson, U.S. Pat. No. 2,687,803; Roberts, U.S. Pat. No. 2,737,348; Breakiron, U.S. Pat. No. 3,322,275; and Barthelemy, U.S. Pat. No. 3,308,948. A modified system in which the grounded conductive roll is comprised of alternating disks of conductive and nonconductive material is disclosed in Payne, U.S. Pat. No. 994,870. Another modified system in which the roll is provided with a surface layer of highly resistive or semiconductive metal oxide is described in Fraas, U.S. Pat. No. 3,012,668. All of these systems depend on differences in the conductivities of the different materials to achieve separation.
In the recycling of waste materials it is necessary to separate valuable constituents from other materials in the refuse mixture. For example, glass is considered a valuable component of typical municipal refuse. Various techniques have been utilized in the prior art for separating glass from refuse. Pierson, U.S. Pat. No. 3,236,604 discloses a refuse recycling scheme in which large pieces of glass are hand picked from the refuse stream and small pieces of glass are removed along with ashes and dirt by screening. Such techniques are extremely inefficient and uneconomic. Rhys, U.S. Pat. No. 3,650,396 discloses a separation of glass from a waste material stream by air classification and optical sorting. In a similar vein, Marsh, U.S. Pat. No. 3,720,380 discloses separating glass from municipal waste by screening the material, passing the screened material through an air cyclone and then sorting out the glass particles with an optical sorter. The use of optical sorting is disadvantageous because the sorting equipment is complex and expensive and because the optical sorting equipment has a very limited capacity.
Bradbury, U.S. Pat. No. 3,941,684 discloses a scrap salvage system in which a conventional high tension separator is utilized to separate plastic and metal. Rhys, U.S. Pat. 3,897,330, Marsh, U.S. Pat. No. 3,945,575 and Webb, U.S. Pat. No. 4,116,822 disclose refuse recycling systems in which conventional high tension separators with conductive drum electrodes are utilized to separate glass from waste material. Conventional high tension separation can be used to produce a glass concentrate, but the efficiency of separation is still less than could be desired. The refuse must be reduced to a fairly small particle size if substantial amounts of refuse material are to be processed through the separator, and the refuse must also be dried to a very low moisture content.
The problem encountered in separating glass from other materials in a refuse mixture, especially stones, lies in the fact that both the stones and the glass have a very high electrical resistivity so that a conventional high tension separation which depends upon differences in electrical conductivity has a difficult time discriminating the two types of materials. Also stones and glass typically have similar densities, so that separation techniques which depend upon differences in density are not effective to achieve high quality separations. Appreciable amounts of non-glass material remaining in the glass concentrate from a conventional high tension separator must be removed by a subsequent sorting operation. Moreover, conventional high tension separation of glass from waste material requires that the waste material be dried before it is fed to the separator. Typical municipal refuse has an appreciable moisture content ranging up to 15 or 20 percent by weight. Also, treatment steps such as flotation separations which increase the moisture content of the stream of refuse are frequently utilized in refuse recycling systems. Drying both the valuable and nonvaluable components of a refuse prior to effecting a separation involves substantial expenditures of costly energy. It would be desirable if separator efficiency could be increased while at the same time eliminating the need for drying the particulate refuse.